EP0796190A1

EP0796190A1 - Pneumatic brake servo

Info

Publication number: EP0796190A1
Application number: EP95934204A
Authority: EP
Inventors: Jean Pierre Gautier; Ulysse Verbo
Original assignee: Bosch Systemes de Freinage SAS
Current assignee: Bosch Systemes de Freinage SAS
Priority date: 1994-12-09
Filing date: 1995-10-12
Publication date: 1997-09-24
Anticipated expiration: 2015-10-12
Also published as: DE69516014D1; FR2727921B1; FR2727921A1; DE69516014T2; ES2143653T3; WO1996017759A1; EP0796190B1; JPH11503684A

Abstract

Pneumatic brake servo comprising a body (10) having an axis of symmetry (X-X'), sealingly separated by a moveable partition (12) into a front chamber (14) permanently linked to a low pressure source and a rear chamber (16) selectively linked to the front chamber (14) or to a high pressure source by a three-way valve (30) actuated by a control rod (28) which engages, via the front face of a plunger (26), with the rear face of a thrust rod (82) integral with a reaction plate (80). An auxiliary housing (32) integral with said moveable partition (12) projects into the front chamber (14). An auxiliary piston (34) sealingly divides the interior of the inner auxiliairy housing (32) into a front volume (36) and a rear volume (38), the auxiliairy piston (34) being mounted stationary in relation to the body (10), the front volume (36) being permanently linked to the rear chamber (16) and the rear volume (38) being permanently linked to the front chamber (14). According to the invention, the brake servo further comprises at least one member integral with the front (42) and rear (44) walls of the body (10) for transmitting the reaction force (50, 100) which passes through the body (10) parallel to the axis of symmetry (X-X') of said brake servo.

Description

PNEUMATIC BRAKE SUPPORT SERVOMOTOR

The present invention relates to pneumatic servomotors, of the type which are used to provide braking assistance for motor vehicles.

Such servomotors are well known and commonly used in the automotive technique. They conventionally comprise an envelope fixed by its rear wall to the bulkhead separating the engine compartment from the passenger compartment, so as to be able to be actuated by a brake pedal located in the passenger compartment and to actuate the primary piston of a master -cylinder located in the engine compartment and connected by a hydraulic circuit to the vehicle brakes, the master cylinder being fixed on the front wall of the casing of the booster.

The envelope is separated in a leaktight manner by a movable wall structure into a front chamber permanently connected to a low pressure source, and a rear chamber selectively connected to the front chamber or to a high pressure source by valve means. three-way, actuated by a control rod connected to the brake pedal, and which is capable of bearing, via the front face of a plunger, on the rear face of a push rod integral a reaction disc and actuating the primary piston of the master cylinder.

In order to increase the assistance effort provided by such servomotors, double or tandem servomotors have been developed, consisting schematically of two servomotors arranged in series. These tandem actuators then have a relatively large size, which limits their use to certain vehicles where the space available in the engine compartment is sufficient. To overcome this drawback, attempts have been made to develop, for example in document DE-A-3 343 160, servomotors comprising an additional chamber, of reduced volume, so as to increase the assistance force supplied by the servomotor without significantly increasing the external dimensions. There is known, for example from document EP-B-0 395 461, a booster corresponding to the preamble of the main claim, and in which an auxiliary housing is integral with the movable wall and projects into the front chamber. An auxiliary piston internally sealingly divides the auxiliary housing into a front volume and a rear volume. The auxiliary piston is mounted fixed relative to the casing, and the front volume is permanently connected with the rear chamber and the rear volume is permanently connected with the front chamber.

The disadvantage of these additional chamber servomotors lies in the fact that, transmitting increased assistance forces, they are subjected to all the more significant deformations in the axial direction. It therefore follows that, when the actuator is actuated for a braking action, the axial distance between the front and rear walls of the actuator varies, which results in an elongation of the stroke of the brake pedal.

The present invention therefore aims to provide a booster having an additional chamber, and whose rigidity in the axial direction is increased, so that the stroke of the brake pedal is constant. For this purpose, according to the present invention, the booster of the type recalled above further comprises at least one element for transmitting reaction force passing through the casing right through parallel to the axis of symmetry of the booster and integral with the front and rear walls of the enclosure.

Other objects, characteristics and advantages of the present invention will emerge more clearly from the description which follows of an exemplary embodiment given by way of illustration, with reference to the appended drawings in which: Figure 1 is a side view, in section longitudinal, of a pneumatic brake booster, produced in accordance with the present invention, and FIG. 2 is a side view, in longitudinal section, of a pneumatic brake booster, produced in accordance with a second embodiment of the invention.

The Figures show a pneumatic brake booster intended to be placed in the usual way between the brake pedal of a vehicle and the master cylinder controlling the hydraulic braking circuit of this vehicle. By convention, the part of the servomotor facing the master cylinder is called "front" of the booster and the part facing the brake pedal is the "rear" part of the booster. In the Figure, the front is thus on the left and the rear on the right.

The booster shown in the Figures comprises an outer shell 10 in the form of a shell, having a symmetry of revolution about an axis XX ′.

A movable wall structure 12 sealingly delimits inside the envelope 10 a front chamber 14, permanently connected by a non-return valve 15 to a vacuum source and a rear chamber 16. The movable wall 12 is associated with a flexible unwinding membrane 18 made of elastomer, the inner peripheral edge of which is received in a sealed manner by means of an inner bead in a hollow assistance piston 20 disposed along the axis XX 'of the booster, and whose outer peripheral edge is fixed tightly on the outer casing 10.

The hollow piston 20 extends towards the rear in the form of a tubular part 22 which passes tightly through the rear wall of the casing 10. A compression spring 24 interposed between the piston 20 and the front wall of the envelope 10 normally holds the piston

20 in the rear rest position illustrated in the Figure, in which the rear chamber 16 has its minimum volume and the front chamber 14 its maximum volume. In the central part of the movable wall located in front of the rear tubular part

22, the piston 20 has a bore in which a plunger 26 is slidably received. The front end of a control rod 28 of the booster, also arranged along the axis XX ', is pivotally mounted in a blind bore of the plunger 26, and its rear end, which projects outside the tubular part 22, is controlled directly by the vehicle brake pedal (not shown).

In known manner, the plunger 26 controls the operation of a three-way valve 30, selectively putting the rear chamber 16 in communication with the front chamber 14 or with a high pressure source, such as for example the atmosphere present at l rear of the tubular part 22. An annular auxiliary housing 32 is integral with the movable wall 12 and projects into the front chamber 14. An annular auxiliary piston 34 sealingly delimits inside the housing 32 a front volume 36 and a rear volume 38. The sealing is ensured by a flexible unrolling elastomer membrane 40, the outer peripheral edge of which is tightly fixed to the housing 32.

According to the present invention, there is provided a reaction force transmission element between the front wall 42 and the rear wall 44 of the casing 10 of the booster.

According to the embodiment of Figure 1, this force transmission element consists of a tie rod, integral with the annular auxiliary piston 34, or formed in one piece with it as shown. More specifically, the tie rod 50 consists for example of a tube, the two ends of which are tapped. In the rear end is screwed a first part of a stud 52, formed with a median bulge 54 disposed inside the casing 10 and in abutment on the internal face of the rear wall 44, the second part 56 of the stud 52 passing through an opening in the wall 44, the edges of which are crimped onto the stud. A seal could advantageously be inserted between the bulge 54 and the wall 44.

Similarly, the first part of a second stud 58 is screwed into the front end of the tie rod 50, the second part 60 of the stud 58 sealingly passes through an opening in the front wall 42, for example using a tubular rubber plug 62 as shown. The plug 62 is also applied in a sealed manner to the tie rod 50.

In the piston 34, an annular groove 64 is formed around the tie rod 50 for sealingly receiving a bead of the membrane 40. Likewise, the membrane 18 of the movable wall structure 12 is formed with an opening whose edges form a bead 66 tightly fixed around the rear end of the tie rod 50.

Axial grooves 68 are formed in the thread of the rear end of the tie rod

50, over a length greater than the length of the first part of the stud 52, and radial openings 70 are made in the rear end of the tie rod 50, behind the fixing of the bead 66, and open into the grooves 68, for communicate the rear chamber 16 of the booster with the volume 72 inside the tubular tie rod 50.

Radial openings 74 are also made in the tubular tie rod 50, in front of the groove 64 for fixing the bead of the membrane 40, to make the volume 72 communicate with the front volume 36 of the auxiliary housing 32. Openings 76 are made in the wall of the auxiliary box 32, to communicate the front chamber 14 of the booster with the rear volume 38 of the auxiliary box 32

Finally, a lip seal 78 is disposed in an opening formed at the front of the auxiliary housing 32, to ensure a sliding seal between this housing and the tie rod 50. A nut 79 may advantageously be screwed onto the second part 60 of the stud 58, on the one hand to keep the two parts 42 and 44 forming the casing 10 assembled and thus to facilitate the handling and the storage of the booster, and on the other hand to precisely determine the axial distance at rest between the front 42 and rear 44 faces of the booster. The booster thus assembled is fixed to the bulkhead (not shown) separating the engine compartment of the vehicle from the passenger compartment, by means of a nut screwed by the passenger compartment onto the second part 56 of the stud 52 passing through an opening in the bulkhead. A master cylinder (not shown) is fixed to the front of the actuator by means of a nut screwed on the second part

60 of the stud 58 passing through an opening formed in a fixing flange of the master cylinder.

When the entire actuator is in the rest position, the control rod 28 as well as the plunger 28 are biased towards the rear, so that the three-way valve normally establishes communication between the two chambers 14 and 16 of the actuator . These chambers communicating permanently with the volumes 38 and 36 respectively of the auxiliary unit 32, the entire servomotor is subjected to the pressure supplied by the vacuum source via the valve 15. When the driver of the vehicle presses on the brake pedal, this results in a forward movement of the control rod 28 and of the plunger 26, which controls the operation of the three-way valve 30. Air at atmospheric pressure is then admitted into the chamber rear 16 and the front volume 36. The pressure difference with the front chamber 14 and the rear volume 38 respectively, in which the pressure of the vacuum source still prevails, generates an assistance force transmitted by the hollow piston on a disc 80, also receiving the actuating force on the control rod 28 via the plunger 26.

The reaction disc 80 is integral with a push rod 82, which actuates the primary piston of the master cylinder, which thus increases the hydraulic pressure in the braking circuit to which it is connected. The actuation of the master cylinder generates a reaction force on the fixing flange thereof. This flange being fixed on the stud 58, itself integral with the tie rod 50, fixed by the stud 52 on the bulkhead of the vehicle, ia reaction force created by the actuation of the master cylinder is thus transmitted directly to the bulkhead of the vehicle by the pulling 50, without being transmitted by the casing 10 of the booster.

It is therefore possible to produce the casing 10 of the booster in a finer or lighter material, since this casing is only subjected to the only stress generated by the differential pressure between its internal volume and the external atmosphere.

There is shown in Figure 2 a variant of the embodiment which has just been described. In FIG. 2, the same elements as those in FIG. 1 are given the same reference signs, and they will not be described again in detail. According to this variant, the tie rod is compressible in the axial direction, so as to be able to transmit the reaction force from the master cylinder to the bulkhead, without transmitting forces on this bulkhead in the event of a vehicle collision, and thus increase passive safety of the vehicle equipped with such a servomotor.

The tie rod 100 shown in FIG. 2 consists of a cable 102 at the ends of which are crimped end pieces 104 and 106. Each piece 104 and 106 is provided, at its end, with a threaded portion 108 and 1 10 respectively, passing through an opening in the wall 42 and 44. The end piece 106 is formed with a median bulge 112, disposed inside the envelope 10 and in abutment on the internal face of the rear wall 44, the edge of the opening in the wall 44 being crimped on the end piece 106. A seal could advantageously be interposed between the bulge 112 and the wall 44. The annular auxiliary piston 34 is integral with a tube 114, surrounding the tie rod 100, and sealingly passing through the auxiliary housing 32, by means of a lip seal disposed in an opening formed at the front of this housing 32. The tube 1 14 is provided with openings 74 and 70 as in the previous embodiment, to communicate the rear chamber 16 of the booster with the front volume 36 of the auxiliary housing 32. Likewise, the tubular plug 62 seals the passage of the end piece 104 through the wall 42, and from the inside of the tube 114 relative to the front chamber 14 of the booster. The tube 1 14 has no other function than ensuring the tightness of the cable passage

102 through the front face of the auxiliary box 32, and communicating the rear chamber 16 of the booster with the front volume 36 of the auxiliary box, the transmission of the reaction force due to the actuation of the master cylinder being ensured by the cable 102 of the tie rod 100. The tube 114 can therefore be made of a relatively light material, such as a plastic material.

With such an arrangement, when the actuator is subjected to compression stress, due for example to a frontal collision of the vehicle, the tube 1 14 can break, so that no force towards the rear is transmitted from the wall front 42 of the servomotor to the vehicle bulkhead. The consequences of a collision are therefore reduced for the driver of the vehicle.

Of course, the invention is not limited to the embodiments which have been described, but it is on the contrary capable of receiving numerous modifications which will appear to a person skilled in the art, without departing from the scope of the appended claims. We can for example provide that the compressible tie rod is of the telescopic type, and consists for example of two tubes threaded one inside the other. The tie rod 50 of Figure 1 can thus consist of two tubes, a rear tube fixed to the rear wall 44 of the booster and secured to the auxiliary annular piston 34, and a front tube fixed to the front wall 42 of the booster, the front tube comprising an external radial shoulder cooperating with an internal radial shoulder of the rear tube to ensure the transmission of the reaction forces, a seal being disposed between the two tubes to isolate the interior space of the tie rod from the front chamber of the booster, the front tube being able to slide in the rear tube under the effect of an external stress following a collision, so as to avoid any transmission of forces from the front wall of the booster to the bulkhead of the vehicle.

Claims

1 - Pneumatic brake booster, comprising a casing (10) having an axis of symmetry (XX), separated in leaktight manner by a movable wall structure (12) in a front chamber (14) permanently connected to a low pressure source, and a rear chamber (16) selectively connected to the front chamber (14) or to a high pressure source by a three-way valve means (30) actuated by a control rod (28) capable of bear, via the front face of a plunger (26), on the rear face of a push rod (82) secured to a reaction disc (80), an auxiliary box (32) being integral with the movable wall (12) and projecting into the front chamber (14), an auxiliary piston (34) internally sealingly dividing the auxiliary housing (32) into a front volume (36) and a rear volume (38 ), the auxiliary piston (34) being mounted fixed relative to the casing (10), the volume before (36) being permanently connected with the rear chamber (16) and the rear volume (38) being permanently connected with the front chamber (14), characterized in that it also comprises at least one force transmission element reaction (50,100) passing through the envelope (10) right through parallel to the axis of symmetry (XX) of the booster and integral with the front (42) and rear (44) walls of the envelope (10).

2 - Pneumatic booster according to claim 1, characterized in that the reaction force transmission element (50) is a tie rod (50) secured to the auxiliary piston (34).

3 - Pneumatic booster according to claim 2, characterized in that the tie rod (50) sealingly crosses the front face of the auxiliary housing (32).

4 - Pneumatic booster according to claim 3, characterized in that the tie rod (50) is tubular over at least part of its length, and comprises at least one first opening (70) communicating the interior space (72) of the tie rod (50) with the rear chamber (16) and at least one second opening (74) communicating the interior space (72) of the tie rod (50) with the front volume (36).

5 - Pneumatic booster according to claim 1, characterized in that the reaction force transmission element (100) is compressible in the axial direction (X-X).

6 - Pneumatic booster according to claim 5, characterized in that the compressible reaction force transmission element (100) has over at least part of its length a cable (102). 7 - Pneumatic booster according to claim 6, characterized in that the cable

(102) is disposed inside a tube (1 14) sealingly passing through the front face of the auxiliary housing (32).

8 - Pneumatic booster according to claim 5, characterized in that the compressible reaction force transmission element comprises at least two tubes threaded one inside the other to form a telescopic tie rod.